Show Fontawesome-Icons on Smartphones

I have added the Icons from Fontawesome to my navigation but the Icons are not shown on smartphones.
How can I activate the icons for smartphones?
Thank you in advance!

For now I have changed

@media only screen and (max-width: 991px) {
	.main-navigation a {
		float: none;
	}
	/*.main-navigation li::before {
		content: '';
	}*/
}

in the CSS-file.
Is there a better way to do it?

Hi, as you noticed font awesome icons are hidden for mobile display. Since they are hidden with CSS, that would be proper way to show them. It would be better to use child theme and then make changes, but actually you can use plugin to apply custom css code and leave main style.css file intact.